There are a variety of instruments available for measuring the properties of a sample. For example, an oximeter is used for measuring the amount of saturated hemoglobin in tissue capillaries of a person, and a refractometer is used for determining the concentration of a dissolved substance in a liquid sample.
SpO2, which has been viewed as the ‘fifth vital sign’, represents blood oxygen saturation. Medical professionals can detect hypoxemia, i.e. deficiency of oxygen, by monitoring a patient's SpO2. Values between about 95-100% are considered normal; for those below this level indicate hypoxemia.
Pulse oximetry is a technique for measuring the SpO2 of a patient. This parameter is obtained from the patient's arterial oxygen saturation, or SaO2, which is a percentage of oxygenated arterial hemoglobin presents in the patient's blood.
In general, functional hemoglobin molecules can bind with up to four oxygen molecules to yield ‘oxygenated’ hemoglobin (HbO2). A hemoglobin molecule bound with less than four oxygen molecules is classified as ‘reduced’ hemoglobin (Hb). Conventional pulse oximeters use algorithms that assume only HbO2 and Hb are present in the blood, and measure SpO2 from the ratio of oxygenated hemoglobin to the total amount of hemoglobin (both oxygenated “HbO2” and reduced “Hb”) according to equation (1):SpO2=HbO2/(HbO2+Hb)  (1)
HbO2 and Hb levels appear can be measured optically in different absorption spectra, e.g. visible-red and infrared regions, respectively. Conventional pulse oximeters carry light sources that radiate in the visible-red (λ, of ˜660 nm) and infrared (λ, between 900 and 950 nm) spectral regions. They also carry a photodetector for measuring a portion of radiation at each mentioned wavelength that transmits through the patient's pulsating blood.
At 660 nm, for example, Hb absorbs about ten times as much radiation as HbO2, whereas at 925 nm HbO2 absorbs about two times as much radiation as Hb. Detection of transmitted radiation at these wavelengths yields two time-dependent waveforms, each called a photoplethysmogram (PPG), which are used by an oximeter to solve for a SpO2 value as defined in equation (1) above.
Specifically, the oximeter processes two PPG waveforms, one is measured with red (RED(PPG)) and the other one is infrared (IR(PPG)) wavelengths to determine time-dependent AC signals and time-independent DC signals. The AC component is mainly caused by pulsatile changes in arterial blood volume. The frequency of the AC component is equivalent to the patient's heart beat rate. Therefore, the heart beat rate can be estimated directly from counting the frequency of the AC component. The term “AC signals”, refers to a portion of a PPG waveform that varies relatively rapidly with time, e.g. the portion of the signal originating by pulsations in the patient's blood. The term “DC signals”, in contrast, is a portion of the PPG that is relatively invariant with time, e.g. the portion of the signal originates from scattering off of components such as bone, skin, and non-pulsating components of the patient's blood.
Separation of AC and DC signals is typically done with both analog and digital filtering techniques that are well-known in the art. During pulse oximetry, a normalized “R value” is typically calculated from AC and DC signals using equation (2), below:R=(Red(AC)/Red(DC))/(IR(AC)/IR(DC))  (2)
R, represents a ratio of Hb to HbO2. It equates an actual SpO2 value, which ranges from 0%˜100% O2, to an empirical relationship that resembles a non-linear equation. Above about 70% O2, this equation typically yields values that are accurate to a few percent. Measurements below this value, while not necessarily accurate, still indicate a patient who is in need of medical attention.
A refractometer is utilized by emitting a light beam through a sample solution and observing the refracted angle with a preset scale. The amount of the dissolved substances in the sample solution changes the optical properties of the solution in terms of refractive index. When the light passes from air to the sample solution, the speed of light becomes slower and its direction is refracted. As the amount of substance dissolved in the solution is increased, the speed of light becomes slower such that the refracted angle changes more. Specifically, in a refractometer, light from a light source is incident on a sample of index that is smaller than a prism index. The light source is diffused so rays are incident on the sample at all angles, up to a grazing incidence. In particular, the ray at the grazing incidence is refracted into the prism at a critical angle, and has the smallest angle of refraction of all other rays at a light exiting face of the prism. The result of this is that the ray at the critical angle defines a shadow boundary when looking through a telescope at the light leaving the prism. By measuring the angle between the shadow boundary and the normal to the light exiting face of the prism, the refractive index of the sample may be calculated.
Both oximeter and refractometer instruments need a plurality of light sources with at least one wavelength, complicated optical elements, and at least one detector for analytical applications, a computing device is also needed to be installed with at least one testing program. To execute measuring procedures, it includes triggering light beams, determining the signal levels, calculating test results, and outputting the testing results, etc. Therefore, the existing instruments are normally bulky, hard to operate, and expensive. Generally, each of these instruments is customized for a specific testing, or for a single-purpose testing.